How do I know what's running?

Using Windows Task Manager (WinXP) I noticed my available physical RAM was a bit less than I expected so I clicked on the current processes and was shocked to see lots of stuff running I wasn't aware of. The biggest problem being that I don't know what 70% of them are.

Is there a good way to find out what these little apps are and if I really need them? Furthermore, if some are not necessary is there a universal way to see that they don't automatically run in the future? -or will I need to attack each one individually?

I usually look stuff like that up on Yahoo. If it annoys me, chances are it has annoyed someone else in the past. Usually there are removal instructions. Generally these have to be dealt with on a case-by-case basis. Many of these are things you actually want to be there, and you will learn this when you look them up.